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Making sense of Flow Numbers
Simply looking for the highest "peak" flow number is an all too common mistake which has lead to less than desirable performance. Heads with large runner volumes tend to flow well at high lifts. However if your cam has 0.500" peak lift, then it makes no sense to select a head which flows great from 0.600" to 0.700".

In general, the best overall performance is obtained from a cylinder head which has the most "average flow" within the lift range of your cam shaft. A valve spends more time at mid-lift (between 0.200 and 0.500 thousandths) and relatively little time at peak lift. A head which shows good flow in the mid-lift range will outperform, at the track and on the dyno, a head with simply a higher peak lift number. Brian Tooley, President of Total Engine Airflow, and one of the principle designers of the Holley Systemax head, states, "The objective is to get mid-lift airflow as big as possible. Peak flow numbers are not important."

It is wise to look for the best average mid-lift flow numbers (refer to Table 1 Intake Flow). Two heads clearly stand out. The TFS Twisted Wedge flows an astonishing 199cfm average flow between .200" and .500" lift. The AFR also flows an impressive 194cfm in the mid range average. Discounting the larger 2.02" TFS valves and revised valve angles, the AFR cleary is the top head in the Street class. Compared to the stock 5.0L engine which averaged under 130 cfm, the AFR delivered a 30% increase (about the same as the increase you can expect in horsepower). The Brodix 5.0 head came in second at 186 cfm.

 

Table 1: Intake Flow (CFM 28" H20)
lift (in.) TFS* AFR Brodix Holley Edel GT40x World Jr. Stock
.100 62 60 61 56 56 56 54 48
.200 131 125 125 119 114 105 105 92
.300 193 188 175 166 166 154 141 130
.400 230 226 215 200 200 192 170 141
.500 242 238 230 219 214 210 186 155
.600 242 248 233 228 215 224 195 158
Avg. 199 194.3 186.6 176 173.5 165.3 150.5 129.5
*TFS 2.02" intake valve, revised valve geometry.

Choosing a New Head
There is no question that any of these after market street heads will outperform the stock heads. The most important factor in choosing a new head is performance for your engine. Cost, reliability, and ease of installation are also issues but there is little difference between manufacturers in terms of reliability and installation issues, “bolt on” installation is the norm, and pricing is fairly even - in a range of $995 to $1,300. The TFS heads run at the low end of that range, while the AFR's run at the higher end due to the CNC port work from the factory.

Performance from either of the top heads is impressive. Our 1992 Mustang project "Green Machine" with a stock cam, mildly ported Cobra intake, and TFS heads ran 12.50's at nearly 110 mph in quarter mile testing.

However, since the TFS head uses a 2.02” intake valve and modified valve angles it sits on the edge of the “street” and “strip” categories for our comparison. The rotated valves in the TFS heads can cause piston to valve interference problems when using cams with increased lift and duration with stock or domed pistons. In the ‘92 LX Mustang a cam with 220 degrees duration and 0.520” intake lift fit without any clearance issues. Larger cams will require flycutting the pistons.

As for my two favorites, each achieves its excellent performance in a different way. The Trick Flow head uses a revised valve angle and centerline to minimize valve "shrouding", a problematic condition on small block Ford heads whereby the air flow around the valve is restricted due to the valve being too close to the edge of the combustion chamber and cylinder bore walls. The AFR heads use five-axis CNC porting after the casting process to deliver immaculately shaped ports and runners, while retaining the stock valve geometry. This is an advantage for those wanting to install large cams without dealing with special pistons. This is a great head as a “bolt on” to a stock 5.0L while having plenty of room to grow if your power demands increase.

For our most recent project, the decision came down to trying something new. We've had good experience with TFS heads in the past, but having never played with AFR heads seemed to be reason enough to give them a shot. In a future article we'll detail the installation of the AFR heads on a 5.0L Cobra engine, and report the track results. Stay tuned! F/M

Exhaust Flow (CFM 28" H20)
lift (in.) AFR TFS GT40x Edel. Brodix Holley World. Jr. Stock
.100 52 48 49 44 58 57 47 41
.200 100 97 105 91 93 95 101 78
.300 142 134 141 132 130 126 125 95
.400 168 161 158 160 155 152 140 102
.500 181 178 162 171 172 166 148 106
.600 189 189 167 176 183 176 154 105
Avg. 148.7 142.5 141.5 138.5 137.5 134.7 128.5 95.3


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We're stacking the AFR up against the rest of the Small Block Ford heads we tested in our original shootout article.
 

The AFR head features 165cc, CNC ported, intake runners.
 

AFR 165 combustion chamber (58cc) and bowl area. Valves are 1.900" intake and 1.600" exhaust.
 

Trick Flow Twisted Wedge heads utilize 15 deg. intake valve centerline / 17 deg. exhaust valve centerline (stock is 20 deg. for both valves.)
 
 



























 

 

Sources:
www.airflowresearch.com
www.trickflow.com
www.worldcastings.com
www.brodix.com
www.fordracing.com
www.holley.com
www.edelbrock.com


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